EL devices which utilize light emission under application of an electric field show high self-distinguishability due to the self-emission and exhibit excellent impact resistance since they are completely solid devices. Therefore, EL devices have been attracting attention for application as light emitting devices in various types of display apparatus.
The EL devices include inorganic EL devices in which an inorganic compound is used as the light emitting material and organic EL devices in which an organic compound is used as the light emitting material. Organic EL devices have been extensively studied for practical application as a light emitting device of the next generation because the applied voltage can be decreased to a great extent, the size of the device can be reduced easily, consumption of electric power is small, planar light emission is possible and three primary colors are easily emitted.
As for the construction of the organic EL device, the basic construction comprises an anode/an organic light emitting layer/a cathode. Constructions having a hole injecting and transporting layer or an electron injecting layer suitably added to the basic construction are known. Examples of such constructions include the construction of an anode/a hole injecting and transporting layer/an organic light emitting layer/a cathode and the construction of an anode/a hole injecting and transporting layer/an organic light emitting layer/an electron injecting layer/a cathode.
In the development of organic EL devices which can be practically applied, various studies to obtain a light emitting element having a long life and emits light at a high efficiency have been conducted. However, an element having a longer life and emits light at a higher efficiency is desired so that consumption of electric power can be further reduced.
For example, in International Patent Application Laid-Open No. 98/08360 and U.S. Pat. No. 5,853,905, elements in which a mixed layer of an amine derivative having the electron transporting property and an energy gap of 3 eV or greater and an aluminum complex of 8-hydroxyquinoline (Alq) as an electron transporting compound is used as the light emitting medium are disclosed. Since the energy gap of Alq is 2.7 eV, holes and electrons are recombined in Alq having a lower energy gap and light is emitted in this light emitting medium. Since Alq itself has a small quantum yield of fluorescence, the efficiency is enhanced by adding a light emitting dopant such as coumarine and rubrene.
However, the technology disclosed in the above references has a drawback in that the life of the device cannot be increased to the desired value. In other words, in general, few organic materials which simultaneously achieve excellent transportation of electrons and excellent durability under an electric current can be found. It is confirmed that Alq is degraded when holes are injected into Alq although Alq exhibits excellent durability in transportation of electrons. In the case of the above light emitting medium, the energy gap of the hole transporting compound Eg1 and the energy gap of the electron transporting compound Eg2 has the relation: Eg1>Eg2. Therefore, holes tend to be injected into Alq having a smaller energy gap and the life of the device cannot be increased to the desired value. Although it is known that durability of Alq can be improved by addition of a light emitting dopant such as coumarine and rubrene, further improvement has been desired.